Digital Q control for enhanced measurement capability in cantilever-based instruments

ABSTRACT

A digital system for controlling the quality factor in a resonant device. The resonant device can be any mechanically driven resonant device, but more particularly can be a device that includes a cantilever within its system, such as an atomic force microscope. The quality factor can be digitally controlled to avoid noise effect in the analog components. One of the controls can use a direct digital synthesizer implemented in a way that provides access to the output of the phase accumulator. That output is a number which usually drives eight lookup table to produce a cosine or sign output wave. The output wave is created, but the number is also adjusted to form a second number of the drives a second lookup table to create an adjustment factor. The adjustment factor is used to adjusts the output from the cosine table, to create an adjusted digital signal. The adjusted digital signal than drives a DA converter which produces an output drive for the cantilever.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. Ser. No. 11/621,914filed Jan. 10, 2007, now U.S. Pat. No. 7,387,017, which is a divisionalapplication Ser. No. 10/926,787 filed Aug. 25, 2004, now U.S. Pat. No.7,164,445, which claims benefit of the priority of U.S. ProvisionalApplication Ser. No. 60/497,761 filed Aug. 25, 2003 and entitled“Digital Q Control for Enhanced Measurement Capability inCantilever-Based Instruments”.

BACKGROUND

Any mechanically driven resonant system has an inherent “quality factor”or “Q”, that defines some aspect of the way the resonant system reactsto stimuli. The quality factor of certain resonant systems can becontrolled and/or adjusted, for example electronically.

One exemplary mechanically-driven resonant system that can beQ-controlled is the cantilever portion of an scanning probe forcemicroscope. When the microscope is operating in its AC mode, the drivingamplitude and driving phase of the device can be adjusted. The effect ofsuch an adjustment is to make the cantilever system behave as if it hada higher or lower Q then would naturally occur within the system.

FIG. 1 illustrates how an analog based implementation of Q control canbe carried out. Circuit 100 is part of a control system for an atomicforce microscope. A cantilever 105 has a tip 110 that contacts an itemof interest. The cantilever is driven in AC mode via a piezoelectricbased actuator 115. In a system with an optical detector, a laser orother optical source 120 projects a laser beam 122 on the cantilever. Areflection 124 based on the projected laser beam is reflected by amirror 125 to a detector 130 that produces an output signal indicativeof the position of the cantilever tip. For example, the detector mayinclude, as shown, a split position detector 132 that detects deviationfrom its center. The output signal 133 of the detector represents theamount by which the cantilever tip has changed position. The signal 133is sent to a lock-in amplifier 135 which also receives the output of afunction generator 140 that drives the actuator 115.

In order to change the effective Q of the system, the output signal 133is phase shifted by a variable phase shifter 145 which produces a 90degree phase shift, and then multiplied by an amplitude gain by gainamplifier 150. The amplitude gain may be positive in order to damp theQ, and negative to enhance the Q. The resultant Q adjusting signal 155is added by an adder 160, to the driving wave formed by the functiongenerator 140.

This analog phase shifting circuit includes analog components which maybe frequency dependent. Moreover, the resonant frequency may be based oncharacteristics of the specific cantilever, and the way the cantileveris used. Therefore, changing to a new cantilever may change the resonantfrequency of the system. Also, characteristics of the medium in whichthe cantilever is used, such as in air versus in fluid, will change theresonant frequency of the system. This resonant frequency change must becompensated in the phase shift circuit 145 to insure a 90 degree phaseshift between the signal from the cantilever position detector and thesignal 155 that is added to the drive.

The phase shifter 145 is shown as an adjustable phase shifter. This kindof Q adjustment usually requires changing a manually-adjustable value,to change the circuit values of some aspect of the analog phase shift.This adjustment is made to ensure a 90 degree phase shift for the newresonant frequency.

Another possible disadvantage of the analog phase shift circuit is thatanalog phase shifters typically operate only over a limited range offrequencies. In order to phase shift a wideband signal, severaldifferent phase shifters may be used in tandem.

The analog implementation also requires a multiplier to effect theanalog gain. For example, this may be a voltage controlled analogamplifier, or a digitally controlled analog amplifier. However, circuitsof this type may add noise to the output signal 133, and thereby corruptthe effectiveness of the Q control.

SUMMARY

The present system teaches a new kind of Q control which is provided inrecognition of the features noted above. A new, digitally-operating Qcontrol is defined. In one aspect, a system is described which allowscantilever resonant frequency changes without requiring an adjustment ofthe circuit. The system may be relatively noise insensitive, and mayoperate without adding any noise to the Q control scheme.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects will now be described in detail with referenceto the accompanying drawings, wherein:

FIG. 1 shows a block diagram of an conventional analog control of Q;

FIG. 2 shows a block diagram of a digital system of Q control;

FIG. 3 shows a block diagram of another digital system of Q control; and

FIG. 4 shows a more detailed diagram of the digital system of Q control.

DETAILED DESCRIPTION

FIG. 2 shows an embodiment where the calculation of vectors to effect Qcontrol is carried out in a processor 220.

The cantilever system 200 produces a signal 205 which is detected in adetector 210. This signal and detection may be optical as in the firstembodiment. The output 215 of the detector is received by a processor220. The processor produces an output 225 which forms the driving outputto the cantilever 200. Certain A/D conversions and D/A conversions areomitted from the diagram of FIG. 2.

In one embodiment, usable with the system of FIG. 2, the processor 220is used to calculate the amplitude and phase of motion of thecantilever. This amplitude and phase is used to calculate a vector thatis added to the normal driving vector of the cantilever. The outputsignal 225 therefore includes a combination of the driving signal ascorrected for the desired Q control.

In this embodiment, the processor 220 may include or be formed from a“direct digital synthesizer (DDS)” for the mechanically driven system200.

A direct digital synthesizer operates by digitally storing the points ofa waveform to be used, in digital format, and then recalling thosepoints to generate the waveform. The rate at which the synthesizercompletes one waveform then governs the frequency. As the phaseadvances, this also corresponds to advances in the waveform. Phase partswith higher numbers represent points that are further along thewaveform.

The digital number representing the phase is held in a phaseaccumulator. This number is increased at regular intervals until thelimit, after which the number resets and starts counting from zeroagain.

The phase is converted into a digital representation of the waveformusing a waveform map. This is a memory which stores a numbercorresponding to the voltage required for each value of phase on thewaveform. This may be a sine or cosine look up table. The digitalnumbers coming from the sine look up table are converted into an analogvoltage using a D/A converter (DAC). This signal is filtered to removeany unwanted signals and amplified to give the required level asnecessary.

The signal update may be recalculated once per interrupt. This requiresrecalculation and re-application of the vector.

The system described above may operate properly, but requires asubstantial amount of complicated vector calculations. This may requiremore processor power or a larger processor to carry out the calculation.

FIGS. 3 and 4 shows an alternative approach. FIG. 3 shows a blockdiagram of the controller for the mechanically driven resonant system,and FIG. 4 shows more details of that controller. The controller is alsodisclosed in our U.S. application Ser. No. 10/288,877, now U.S. Pat. No.7,266,997 the contents of which are incorporated herein by reference.Specifically, this system allows a very simple calculation, in place ofthe complicated vector mathematics. That simple calculation can be donein the time domain.

In the embodiment of FIGS. 3 and 4, the direct digital synthesizer isimplemented in a programmable device such as a field programmable gatearray.

The direct digital synthesizer includes a phase accumulator registershown as 300. That forms an index 302 to cosine lookup table 305 thatproduces the output driving signal 310 which normally drives thecantilever 200. The direct digital synthesizer is configured in a waysuch that access to the output of the phase accumulator becomesavailable. For example, the DDS may be configured in a programmablearray device. In this embodiment, the phase accumulator output 302 isalso sent to a second, cosine lookup table 320, e.g., one which issubstantially identical to the cosine lookup table 305. A digital adder325 is used to add a cantilever phase adjustment value 330 to the lookup index 302. This creates a compensated value 332. That compensatedvalue 332 forms the phase argument to the second cosine table 320.

The compensation operation is simple, since the index 302 simplyrepresents a number, and can be easily phase-adjusted prior to lookup bysimply adding a compensation to the index 302. No vector operationsbecome necessary.

The output 340 of the cosine table is then multiplied by a gainadjustment, using digital multiplier 345. The gain adjustment representsthe damping gain for the Q adjustment.

The output forms a representation 355 that is digital, but in the timedomain. 355 represents the vector that needs to be added to the maindrive signal 310, to achieve the desired damping result. The drivesignal 310 is thereby adjusted by the compensating signal using thedigital adder 360.

The output feeds a D/A converter 365 which forms the output for drivingthe cantilever.

FIG. 4 shows additional details, including the digital system of Qcontrol of FIG. 3 implemented within a field programmable gate array.FIG. 4 shows the cable 400 to the microscope including its differentparts. These parts including the first signals 402 from a cantilever,second signals 404 to the cantilever, as well as signals 406 whichinclude a drive controlling the z position of the cantilever. Signals408 include a signal from an optional cantilever sensor, and passthrough A/D converter blocks 410 before being sent to the digital signalprocessor 412. The field programmable gate array includes filters and adirect digital synthesizer 414 as described above. The output signals420 may be connected to the drive 406 to the cantilever, via thecrosspoint switch 422.

Although only a few embodiments have been disclosed in detail above,other modifications are possible. For example, while this disclosuredescribed the two cosine tables as being identical, they can bedifferent by an amount related to a desired adjustment. Moreover, theycan be other kinds of tables, such as sine tables, or the like.

All such modifications are intended to be encompassed within thefollowing claims.

1. A system, comprising: a driving part, that includes an accumulatortherein which accumulator has an output signal indicative of a numberrepresenting a current phase of driving of a mechanically drivenresonant system; a first lookup table, driven by said number to producean output signal indicative of a current voltage of driving valueassociated with said number; and a phase adjustment circuit, also drivenby said number, to create an adjustment to said output signal, based ona desired phase adjustment amount.
 2. A system as in claim 1, whereinsaid phase adjustment circuit includes digital circuitry.
 3. A system asin claim 1, wherein said phase adjustment circuit includes a first lookup table, which is used to change said number by an amount related to adesired phase adjustment to create a phase adjustment signal.
 4. Asystem as in claim 3, further comprising a second lookup table, drivenby said phase adjustment signal to create an output signal.
 5. A systemas in claim 4, wherein said second lookup table has substantially thesame content as said first lookup table, and creates an output signalindicative of a voltage that corresponds to said phase adjustmentsignal.
 6. A system as in claim 4, further comprising a gain adjustmentpart, which adjusts a gain of said output signal of said second lookuptable to create a gain adjusted signal.
 7. A system as in claim 6,further comprising an adder which adds said gain adjusted signal to saidoutput signal from said first lookup table.
 8. A system as in claim 7,wherein said adder is a digital adder.
 9. A system as in claim 8,further comprising a D/A converter, receiving an output from said adder,and producing an analog drive signal.
 10. A system as in claim 9,further comprising the mechanically driven resonant system, receivingsaid analog drive signal.
 11. A system as in claim 10, wherein saidmechanically driven resonant system includes a movable cantilever.
 12. Asystem as in claim 11, wherein said movable cantilever is part of anatomic force microscope.
 13. A system as in claim 1, further comprisingan adder, which adds together said output signal from said first lookuptable, and said adjustment created by said phase adjustment circuit, toproduce a compensated output signal.
 14. A system as in claim 13,wherein said compensated output signal is digital, and furthercomprising a D/A converter, converting said signal to analog.
 15. Asystem as in claim 13, further comprising a mechanically driven resonantsystem, receiving said compensated output signal.
 16. A system as inclaim 15, wherein the mechanically driven resonant system includes acantilever.
 17. A system as in claim 15, wherein the mechanically drivenresonant system includes an atomic force microscope, and a cantileverportion of the atomic force microscope.
 18. A system as in claim 1,wherein said driving part and said first lookup table are part of adirect digital synthesizer implemented such that output access to saidaccumulator is available.
 19. A system as in claim 18, wherein saiddirect digital synthesizer is implemented in programmable logic.